Switching power supplies are a class of power supplies where power is converted from one form to another at least in part by periodically cycling one or more switches (e.g., transistors) between their fully on and fully off conditions. Switching power supplies may be contrasted with linear power supplies where one or more devices (e.g., transistors) are operated in their linear region as opposed to in their fully on or fully off conditions. Switching power supplies commonly offer a number of advantages over linear power supplies including higher efficiency and/or smaller size.
Switching power supplies, such as DC-to-DC converters, commonly have at least one inductor that is charged and discharged during each switching cycle of the power supply. Such charging and discharging of the inductor causes an alternating current, frequently referred to as ripple current, to flow through the inductor. Although some ripple current must flow through the inductor in order for the switching power supply to properly function, the ripple current generally causes undesirable power losses in the inductor and other components (e.g., switching transistors) of the power supply. Accordingly, it may be desirable to reduce ripple current magnitude in order to reduce power loss in the power supply. Furthermore, in some switching power supplies, output ripple voltage is generated in proportion to the magnitude of inductor ripple current. This ripple voltage is generally not desired, and it may be desirable to reduce inductor ripple current magnitude in order to reduce output ripple voltage.
Buck and buck derived DC-to-DC converters are a class of switching power supplies. In such DC-to-DC converters, an inductor is connected to an output filter of the DC-to-DC converter. The magnitude of ripple current through the inductor is a function of factors including the voltage applied across the inductor, the DC-to-DC converter's switching frequency, the DC-to-DC converter's duty cycle, and the inductor's inductance value. For example, a buck converter's peak-to-peak inductor ripple current magnitude can be expressed according to the following equation under ideal, steady state conditions:
                              Δ          ⁢                                          ⁢          I                =                                            V              on                        ⁢            D                    FL                                    EQN        .                                  ⁢        1            
In EQN. 1, ΔI is the peak-to-peak ripple current magnitude, Von is the magnitude of the voltage across the inductor when it is being charged (i.e., input voltage minus output voltage), D is the duty cycle of the buck converter, F is the buck converter's switching frequency, and L is the inductor's inductance value.
Note that load current does not appear in EQN. 1. Accordingly, in buck and buck derived converters, the inductor's ripple current magnitude is largely independent of the load powered by the converter. Thus, at light load conditions, the relative ripple current magnitude may be significant, and significant power may be lost in the DC-to-DC converter even when it is powering a light load. Such light load power loss may be highly undesirable in applications, such as battery powered portable applications, where battery life must be maximized.
Although ripple current magnitude in a buck and a buck derived converter may be decreased by increasing the inductor's inductance value, as can be observed from EQN. 1, doing so impairs the converter's ability to respond to changing loads, commonly referred to as transient loads, by limiting the speed at which the converter's output current magnitude can change. How a power supply (e.g., a DC-to-DC converter) responds to rapid changes in load is frequently referred to as the power supply's transient response. Thus, the faster a DC-to-DC converter is able to respond to a change in load, the better the DC-to-DC converter's transient response.
In many buck and buck derived converters, it is important that the converter have a good transient response. For example, the converter may be powering a microprocessor that presents large transient loads and yet has stringent voltage regulation specifications. The converter must quickly respond to the transient loads in order to maintain its output voltage within the microprocessor's stringent voltage regulation specifications.
Output capacitors are required to handle a transient load to the extent that the transient load cannot be handled by the buck or buck derived converter. As a rule, the better a converter's transient response, the less output capacitance is required. Thus, if a buck or buck derived converter's transient response is improved (e.g., by lowering its inductance value), the converter's output capacitance requirements may be reduced, thereby reducing cost and/or saving space. Further, if a buck or buck derived converter's transient response is improved, costly and difficult to procure capacitors having a very low equivalent series resistance (“ESR”) may be replaced with lower cost, easier to obtain capacitors having a higher ESR.
Thus, a buck or buck derived converter's efficiency may be improved by increasing its inductor's inductance value. However, doing so degrades the converter's transient response. Alternately, a buck or buck derived converter's transient response may be improved by lowering its inductor's inductance value. However, doing so degrades the converter's efficiency.
Sun et al. have disclosed a saturable inductor intended to improve light load efficiency in a single phase buck converter. See Sun et al., Light Load Efficiency Improvement for Laptop VRs, Applied Power Electronics Conference, APEC 2007, Feb. 25, 2007-Mar. 1, 2007. Sun's inductor is formed on an E-I core with a winding around the center leg. A small cross section of the center leg core material is designed to saturate when the current through the inductor exceeds a predetermined level so that the inductor has one inductance value at heavy loads, and a larger inductance value at loads below the saturation limit to improve light-load efficiency during discontinuous conduction mode operation.